This application is based on and claims priority from Japanese Application No. HEI 2-121738 filed Nov. 20, 1990, the disclosure of which is incorporated herein.
The present invention relates to a light for an endoscope which emits rays of light to illuminate a body cavity.
Generally, a light source for an endoscope includes a stop mechanism for varying the quantity of light, which is emitted therefrom as it propagates toward a light guide, and a control system for controlling the aperture of the stop mechanism.
There are two types of control systems for the stop mechanism. The first is an open loop type in which the stop mechanism is driven by an actuator, and the second is a closed loop type in which either the aperture of the stop mechanism or the quantity of illuminated light is detected, and the detected signal is fed back to an actuator.
The open loop type of control system is arranged such that the actuator for driving the stop mechanism, a control circuit for controlling the actuator, and a setting means for setting the aperture of the stop mechanism are connected in series.
In the closed loop type of control system where the aperture is sensed, the actuator includes an aperture sensor, which senses motion of the actuator corresponding to the opening or closing of the aperture. The sensing signal is fed to the control circuit, which controls the actuator based on the sensing signal.
In the closed loop type control system in which the quantity of illumination light is sensed, a light-quantity sensor is disposed in the light path to sense the quantity of light passing through the stop mechanism. The control circuit computes the actual aperture of the stop using the sensing signal, and compares the computed value with a preset value. When the actual aperture is different from the preset value, the actuator is controlled so that the aperture of the stop mechanism is equal to the preset value.
In the stop control system of the open loop type, there is no feedback of the actual aperture value. Thus, it is rare that the preset aperture is exactly equal to the actual aperture. Therefore, it is very difficult to reliably gain the desired aperture.
On the other hand, the closed loop control system using the aperture sensor actuator can reliably set the aperture of the stop mechanism to the preset aperture. However, this type of the control system is relatively expensive thereby increases the cost of manufacturing the endoscope light source device. Further, only the actuator of the type having the aperture sensor attached thereto can be used for the stop controller. This fact remarkably reduces the freedom in designing the endoscope light source device.
The closed loop control system using the light-quantity sensor disposed in the light path is problematic when the light source is old and the quantity of light emitted therefrom is correspondingly reduced. Under this condition the aperture corresponding to the sensed light quantity is larger than that of a light source which is relatively new and emits a greater amount of light. The sensing signal does not include information regarding the different aperture size. When a preset value in the aperture setting means is changed from a value corresponding to a full closed state to a value corresponding to an opened state, the comparison of the actual aperture with the preset aperture is based on the amount of light emitted by a new light source. Accordingly, when the preset value in the aperture setting means reaches a certain level, the aperture of the stop mechanism is considered to be fully opened even if the preset value is not reached to a value corresponding to a full opened state. The aperture of the stop is inaccurately changed according to the presetting of the aperture.
A stepping motor for stepwise driving the stop mechanism may be used for the actuator in the open loop control system. However, a complicated control circuit is required for controlling the stepping motor resulting in an increased cost to manufacture the endoscope light source.
Further, when using a stepping motor, the count must always start from a completely open or closed aperture position thus consuming additional time to set the stop to the appropriate aperture.